Toxoplasma gondii is a major opportunistic parasitic infectious agent in man and animals. Disease can result from a weak immune response resulting in parasite lysis of host tissue or it can ensue from an inappropriate or overly exuberant immune reaction. The major protective immune mechanism against T. gondii and other intracellular pathogens is that mediated by T helper 1 (Th1) cells, which produce pro-inflammatory cytokines, IFN-gamma and TNF-alpha. Nevertheless, when produced in excess, these protective cytokines can become pathologic. How Th1 cells are self-regulated has not been well understood. Our work from the previous funding period has unraveled a key negative control loop in Th1 responses whereby IFN-gamma induces the production of the anti-inflammatory cytokine IL-10, Th1 cells themselves. Therefore, the current application will determine how IL-10 is regulated and functions to protect the host from self-inflicted immune-injury. Our first aim is to determine what signals and cells program the Th1 cells to produce IL-10 in the first place. Secondly, we will investigate the role of specialized innate cells in driving the differentiation of Th1 cells producing IL-10. Finally, we will elucidate the cytokine and cellular components of the negative feedback loop whereby IFN-gamma induces reactivation of IL-10 in Th1 memory cells. These studies will identify key mechanisms used by the immune system to simultaneously clear microbial pathogens but also prevent host tissue damage. PUBLIC HEALTH RELEVANCE: Human diseases caused parasitic protozoa and other types of intracellular pathogens are major causes of human mortality and morbidity globally. Disease may be caused by tissue destruction by parasites or result from an over-reaction by the immune response. The studies proposed here will advance understanding of how the Th1 response can provide protection against parasites and other pathogens but also prevent or avert collateral tissue damage. Insights obtained from this research will hopefully be useful for management strategies and treatment for chronic infectious diseases.